Tennis pundits predicted that Australia's Ash Barty would come up against reigning champion Naomi Osaka in the fourth round of the Australian Open, but the Japanese phenom has been knocked out in a huge upset at Melbourne Park. 

America's Amanda Anisimova has defeated the four-time grand slam champion 4-6, 6-3, 7-6 on Margaret Court Arena to progress through to the fourth round, where she will face world No. 1 Barty. 

It's Anisimova's first victory over a top 15 player since February 2020. 

The 20-year-old saved two match points against Osaka at 4-5 in the third set, but managed to hold her nerve and force a super tie-break, that she won 10-5. 

The blockbuster Barty-Osaka clash that tennis fans had been touting all week was not to be. 

"I'm speechless," Anisimova said after the gritty win. "I can't stop smiling. I'm just laughing. I absolutely love this. 

"Going into this match I knew I had to be playing sharp if I wanted to give myself a chance. 

"She is an absolute champion so I knew I would have to step up my game and be aggressive. 

"I'm honestly so grateful that I was able to play so well today and get this win. It means a lot. 

"Every single day here is an amazing opportunity. I'm just thinking about having fun and I'm looking forward to my next round." 

Osaka, who won the Australian Open in 2019 and 2021, will drop to outside top 80 on the WTA rankings after this tournament as a result of the premature exit. 

"I know inside I tried the best that I could," she told reporters after the loss. 

"Pretty high level match. She played really well. The pace of her ball surprised me. It was fun to play. 

"I fought for every point. I'm not god, I can't win every match. I just have to take that into account. 

"I grew a lot in this match. Totally different attitude from my loss at the US Open. 

"I'm here right now at the Australian Open, but you never know when it's going to be the last one. Playing in Rod Laver, I don't want to take that for granted. 

"If I'm not seeded at a slam in the future I think the other players are more worried to play against me than I am to play against them." 

Osaka was spotted by the Channel 9 cameras being consoled by members of her team after the nailbiting defeat. 

"You can see there just how much this hurts tonight," Channel 9 presenter James Bracey said. 

"That is hard to watch." 

Anisimova, who is coached by Australian mentor Darren Cahill, will take on Barty on Sunday, with the victor progressing through to the quarter-finals. 

Earlier in the evening, Barty defeated Italy's Camila Giorgi in straight sets 6-2 6-3 on Rod Laver Arena to cruise into the fourth round. 

The Queenslander has only dropped eight games in the grand slam tournament to date, and is yet to lose a set in Melbourne Park. She has held serve in 58 consecutive games. 

But if Anisimova can replicate tonight's performance on Sunday, Barty will have her work cut out for her. 

"My expectation was whoever it would be," Barty told reporters at Melbourne Park on Friday evening. 

"Each match is uncertain, there are no certainties. You have no idea what's going to happen. 

"You just have to navigate the best you can at that given moment and I've done a good job of that this week and now I get to play Amanda Anisimova. 

"I would have loved to have the opportunity to play Naomi. I love to test myself against the very best but in the position that we're in Amanda played a fantastic tournament, she deserves her spot."